STEREOSELECTIVE EPOXIDATION OF ALLYLIC CARBAMATES WITH M-CHLOROPERBENZOIC ACID - THE ROLE OF COOPERATIVE COORDINATION

Stereoselective epoxidations of a series of allylic carbamate methyl esters 1a-d, homoallylic alcohols 1e-h, and acetates 1i-l have been performed using m-chloroperbenzoic acid (m-CPBA) as epoxidizing agent. Throughout, the formation of the three epoxides was favored. This selectivity is probably due to a directing effect of the carbamate group which forms a hydrogen bond to the peracid. However, the threo/erythro isomeric ratio is also dependent on steric interactions and on the possibility of cooperative coordination of the peracid to other suitably positioned functionalities such as allylic methyl ester, homoallylic alcohol, and acetate groups. The results of the present study indicate that the methyl ester is a weaker directing group than the carbamate. However the directing effect of the methyl ester is stronger than that of the homoallylic alcohol and acetate groups. A thermodynamic study of the epoxidation of two epimeric carbamate esters giving considerably different isomeric product ratios (9:1 compared to 2:1) indicates that the order in the transition state structure influences the isomeric ratios.